The present invention relates to an optical track sensing device, and in particular to an optical sensor for detecting optical tracking information on a moving medium.
U.S. Pat. No. 5,991,249 described a method whereby the motion of a moving medium with a grating on its surface can be detected by a sensor consisting of a semiconductor laser and a photodetector. In one example, the moving medium was a floppy disk. This prior art is based on the use of a sinusoidal pattern on the detector to select a particular diffracted wave from the moving grating. By placing the sensor in the self-imaging plane, the need for optical components was reduced. However, in many applications it is necessary to detect the beginning position of the moving grating. In certain applications positional encoding in two dimensions is also required. It is the intention of this application to describe certain improvements to the prior art U.S. Pat. No. 5,991,249, the disclosure of which is incorporated herein by reference.
The present invention reflects the recognition that a very acceptable detected signal can be obtained at other than the self imaging plane of the reflection off a moving medium (or a stationary medium and moving sensor). This provides more flexibility in the placement of the sensor, while still allowing the elimination of optical components by relying on self-imaging instead. In addition, by providing a pattern over the photodetector with a smaller period, higher frequency harmonics are detected, allowing more precise detection of the position of the medium.
In a first embodiment of this invention the period of the detector pattern is selected to detect the higher harmonics of the grating on the medium. Moreover, in this embodiment the placement of the detector can be on planes other than the self-image plane of the grating to increase the resolution in detecting the motion of the grating.
In one version of the first embodiment, the detector contains three light sensitive areas with phase shifts among them for the detection of the motion of a moving medium. This invention recognizes that the differential detection benefit of quadrature signals can be obtained with only three, rather than four, detectors. The invention also applies to a stationary medium, but for simplicity, the discussion herein is directed to the example where the medium moves.
In a second embodiment of the present invention a diffractive cylindrical lens is placed in front of the laser to focus the laser beam to a line on the surface of the moving medium. This line beam is perpendicular to the lines forming the grating. This increases the light collection by the motion detector and reduces noise generated by multiple reflections. In one version of this second embodiment, the period of the detector pattern corresponds to the nth harmonic of the grating. In another variation, the laser beam is focused to a line on the detector. This further improves the light collection at the detector, at the expense of a less sharp image from the medium, which is acceptable in some embodiments.
A third embodiment of the present invention provides for detection of the index marking recorded on the grating. A two element photodetector is used to detect the index. There is no need for a pattern on the photodetector, a simple reflection from the index marking is detected.
A fourth embodiment of this present invention provides an integrated position sensor, which can both detect the motion of the medium and the index marking on the medium. A first laser and detector combination, with a pattern over the detector, detect the motion of the medium by detecting the grating pattern. A second laser and detector combination are mounted offset from the first laser and detector combination, to detect an index marking adjacent the grating on the medium.
A fifth embodiment of the present invention uses a diffracting grating to direct the reflected light to the photodetectors. The grating has the periodic pattern overlayed on it, instead of putting the pattern on the detector itself. This allows the use of a generic detector and reduces the design and manufacturing time for a new pattern. The grating has different periods in different zones, so that the different zones direct the reflected light to different photodetectors.
A sixth embodiment of this invention provides a sensor which can detect motion of a moving medium in two dimensions. Two sets of detector elements are used, each aligned with an axis of one of the two dimensions. Each set of detector elements has its own pattern, corresponding to the grating frequency along its axis on the moving medium. Diffraction gratings can be used to minimize the interference effect of the medium pattern in the other dimension.